Since the ways of combining things are not necessarily logical, when something doesn't work it's unclear whether you don't have the right idea, or just didn't click in exactly the right place. I puzzled over one of the more obvious ones for far too long because my idea of which end of a pointy object should be used wasn't the same as the designer's. I just assumed I didn't have the right pointy object yet. Also, the solution to the last puzzle is incorrect, strictly speaking. I can find one more of each ... can't you?